This application claims priority from German Application No. 100 09 790.1, filed on Mar. 1, 2000, the subject matter of which is hereby incorporated herein by reference.
1. Field of the Invention
The present invention relates to a process for the production of organosilylalkyl polysulfanes.
2. Background Information
It is known that organosilylalkyl polysulfanes such as bis(3-triethoxysilylpropyl)tetrasulfane (DE 2 141 159 and bis(3-triethoxysilylpropyl)disulfane can be used as silane coupling agent or reinforcing additive in rubber mixtures filled with oxides. The rubber mixtures are used, inter alia, for industrial rubber articles and for parts of vehicle tires, in particular for treads (DE 2 141 159, DE 2 212 239, U.S. Pat. No. 3,978,103, U.S. Pat. No. 4,048,206).
Various processes for producing organosilylalkyl polysulfanes are described in the literature. In this respect processes that use industrially easily accessible organosilylalkyl halides represent the most economical and simplest alternative. These organosilylalkyl halides are reacted with ionic polysulfides, whereby the halide functions of two molecules are replaced by polysulfane units by nucleophilic substitution and thereby bonded to one another.
In this process the preparation of the nucleophilic polysulfide presents the most difficulty. Of course, ionic polysulfides can be obtained relatively easily in aqueous phase according to a method known per se by reacting sulfur with alkali sulfide hydrates, alkali hydrogen sulfide hydrates or caustic soda and the resultant aqueous alkali polysulfide solutions can then be reacted with organosilylalkyl halides in a phase-transfer catalytic system to form the analogous polysulfanes (EP 694552, EP 794186, EP 839816). However, with the known process there is always the disadvantage that large proportions of the alkoxysilane starting material are converted into ineffective solid polysiloxanes due to hydrolysis and condensation. The organosilane polysulfides produced according to the known process are also characterised by an unsatisfactory storage stability.
These disadvantages can be avoided by working with anhydrous or virtually anhydrous starting substances in organic solution. U.S. Pat. No. 5,399,739 and EP 705838 disclose processes in which ionic polysulfides are produced by reacting alcoholates with hydrogen sulfide, which in turn are then reacted with sulfur and the corresponding organosilylalkyl halide. The disadvantage of these processes is that hydrogen sulfide gas, which is problematic from the safety and toxicological aspects, and alcoholates, which are not very stable under storage, are used in the production of the anhydrous sulfide.
A technically better solution for producing anhydrous or virtually anhydrous sulfides is to dry alkali sulfides, principally sodium sulfide hydrate, which are commercially available in large quantities. From JP 7228588 it is known that the drying process can be carried out azeotropically as well as in vacuo under heating, and the anhydrous or virtually anhydrous sulfides thus obtained can be reacted with sulfur to form alkali polysulfides. These polysulfides in turn react with organosilylalkyl halides to form the corresponding polysulfanes.
A similar process involving a variation of the azeotropic drying of the alkali sulfide is known from EP 795558.
The disadvantage of these processes however is that a polysulfide first of all has to be produced in a preceding process step from the alkali sulfide and sulfur, and can only then be converted by reaction with the organosilylalkyl halide into the desired polysulfide.
In DE 19651849 a process is described in which the production of the polysulfide already takes place during the drying. The process step involving the polysulfide production can thus be avoided and cost savings can be made, which significantly improves the economy of the process.
Further processes that likewise produce organosilylalkyl polysulfanes from anhydrous or virtually anhydrous ionic sulfides and that avoid a preliminary polysulfide production stage are described in DE 19734295 and EP 949263.
A common feature of all the cited processes is that an anhydrous polysulfide is mixed with a polar organic solvent and the organosilylalkyl halide is added to this solution/suspension. However, highly colored and often also unpleasantly smelling products are formed in such a procedure.
It is an object of the invention to provide a process in which organosilane polysulfides are produced that are only slightly colored and have only a slight unpleasant odor.
The present invention accordingly provides a process for the production of organosilylalkyl polysulfanes of the general formula
R1R2R3SiR4)2Sxxe2x80x83xe2x80x83(I) 
in which
R1, R2, R3, which are identical to or different from one another, denote branched and unbranched alkyl and/or alkoxy groups with a chain length of 1-8 C atoms, preferably 1-3 C atoms, aryl radicals, in particular phenyl, toluyl, benzyl, wherein at least one alkoxy group is present;
R4 denotes a divalent alkylene radical with a chain length of 1 to 8 C atoms, preferably 1 to 5 C atoms, particularly preferably methylene, ethylene, i-propylene, n-propylene, i-butylene, n-butylene, n-pentylene, 2-methylbutylene, 3-methylbutylene, 1,3-dimethylpropylene and 2,3-dimethylpropylene, or xe2x80x94(CH2)nxe2x80x94C6H4xe2x80x94(CH2)nxe2x80x94 where n=1-4;
x is a number  greater than 1, preferably between 2 and 6,
by reacting an organosilylalkyl halide of the general formula
R1R2R3SiR4Xxe2x80x83xe2x80x83(II) 
in which
R1, R R3 and R4 have the meanings given above, and
X is chlorine, bromine or iodine,
and an anhydrous or virtually anhydrous ionic sulfide of the general formula
M+2S2xe2x88x92xe2x80x83xe2x80x83(III) 
in which M+ denotes an alkali metal cation, preferably a sodium or potassium cation, an ammonium ion, an alkaline earth metal cation or a zinc cation,
and elementary sulfur,
which is characterised in that the elementary sulfur and organosilylalkyl halide are placed in a polar organic solvent and the anhydrous or virtually anhydrous ionic sulfide is added to this suspension.
After the reaction the organosilylalkyl polysulfane can be isolated by filtering off the precipitated halide and separating the solvent by distillation.
On account of the susceptibility of the organosilylalkyl halide (II) to undergo hydrolysis, the ionic sulfides (III) must be anhydrous or virtually anhydrous. Virtually anhydrous ionic sulfides (III) are understood to be compounds according to formula (III) containing at most 10 wt. %, preferably 0-5 wt. %, and particularly preferably 0-2 wt. % of water. The virtually anhydrous ionic sulfides (III) can be obtained in various ways:
Reaction of alkali metal alcoholates with hydrogen sulfide (EP 0 705 838).
Reaction of ammonia gas with hydrogen sulfide (DE 26 48 241).
Drying alkali sulfide hydrates (DE 196 10 281, JP 7 228 588 and DE 196 51 849).
In this connection it is unimportant whether the drying of the alkali sulfide hydrates is carried out azeotropically or by heating in vacuo. Preferably, the required ionic sulfide may be prepared according to the process described in DE 196 51 849. The ionic sulfide (III) may, without influencing the reaction yield, be used as a ground powder as well as in the form of small platelets, such as are present in the case of the commercially available alkali sulfide hydrates.
The amount of ionic sulfide (III) necessary for the reaction may be added in one lot or in partial amounts to the suspension consisting of the solvent, organosilylalkyl halide (II) and elementary sulfur. The ionic sulfide (III) may be added continuously or discontinuously.
The sulfur may be added in solid form, for example as a commercially available sulfur powder, or granules, or in molten form.
In order to accelerate the course of the reaction the sulfur may be used in finely divided form, for example as finely ground sulfur powder or as fine droplets of atomised melt.
As organic solvent there may in principle be used all polar solvents in which the ionic sulfide (III) is at least partially soluble, and which do not react with the organosilylalkyl halide (II).
Linear or branched alcohols with 1-8 C atoms, such as for example methyl, ethyl, propyl, butyl or pentyl alcohol, cycloalkyl alcohols with 5-8 C atoms, phenol or benzyl alcohol, may preferably be used as organic solvent.
In order to avoid a transesterification, it may be more expedient to use the alcohol corresponding in each case to the groups R1, R2 and R3. Optionally, it may also be advantageous to use a mixture of these alcohols, for example if different alkoxy groups R1, R2, R3 are present in the compound II.
The molar ratios of the individual reactants to one another are governed by the mean sulfur chain length that is to be established in the organosilylalkyl polysulfane (I) to be produced therewith, and by the residual content of organosilylalkyl halides (II) that is to be present in the end product. Accordingly the molar ratio of the ionic sulfide of the formula (III) to the elementary sulfur that is used controls the mean polysulfane chain length in the end product. For the process according to the invention ionic sulfide:sulfur may be used in a molar ratio of at least 1:0.1, preferably 1:0.8 to 1:5.2.
The molar ratio of ionic sulfide to the organosilylalkyl halide determines the residual content of the starting material in the end product. For the process according to the invention a ratio of ionic sulfide to organosilylalkyl halide of 1:1 to 1:3 may be chosen, preferably a ratio of 1:1.5 to 1:2.2.
The reaction may be carried out with the exclusion of air and water (moisture) in order to suppress or largely avoid the formation of secondary products. The reaction may be carried out at elevated temperature. In this connection it is immaterial for the process according to the invention whether, in order to reach the reaction temperature, the reaction mixture is heated externally or is heated solely by the heat released due to the exothermic reaction. The reaction may be carried out between room temperature and 200xc2x0 C., preferably between 40xc2x0 C. and the boiling point of the solvent that is used. The reaction may be carried out under reduced pressure, normal pressure or slight superatmospheric pressure.
The organosilane polysulfides produced by the process according to the invention have the advantage that they are less colored and have a less unpleasant odor than the known organosilane polysulfides.